My male-to-female tg gf left me just over 3 months ago.  She became attracted to another male-to-female and they've been together since our breakup.  She said it wasn't my fault.  We initially me on a TS dating website.  She was looking for a man and I was looking for a tg.  Her initial concerns were our age difference (15yrs) and that I had self medicated for a month.  She also knew that I cross-dressed and was okay as long it wasn't around her. 

We were together for 20-months.  She said she was comfortable in our relationship, but unfulfilled.  I think she felt that she was hindering my own introspection into my gender dysphoria.  She said that if I ever decided to transition, she support me, but we couldn't remain in a relationship.  Now, as I say, she's with another m-to-f tg. 

I have since started seeing a gender therapist who had referred me to an endo doc.  I've been on low-dose HRT for 6-weeks now.  I've not told my ex-gf as I still have strong feelings for her.  I'm afraid if I tell her it would ruin any chance of getting back together.  Alternatively, if I tell her, I'm not sure if she'll keep her word and support me.  What to do?

Find someone else. You deserve better. She seems confused and you will only end up more hurt then you are now.

Honestly, I think it's time to move on.  You can be friends and have her emotional support, but don't expect a relationship.  She has been seeing someone else for a few months, so it seems like it isn't right to try and rekindle any flame.  If it's meant to be, things will work out in the end.  However, for now, I would let go of your plans on dating her for the sake of everyone involved.